1. Field of the Invention
The invention relates to industrial type noncontact detection systems for ascertaining the presence of an object at a point in a machine system. More specifically, the invention relates to a laser based noncontact detection system for ascertaining the presence or absence of rotating objects or objects in some other continuous repetitive motion. Due to its ability to operate in harsh environments and accommodate different drill bit sizes, it is particularly useful to detect broken drill bits used in the automated routing and drilling of printed circuit boards and to determine whether the tips of such drill bits have been broken or damaged.
2. Description of the Prior Art
The creation of automated manufacturing has produced many benefits to mankind. For example, the manufacture of most consumer and industrial electronic instruments and appliances is largely automated. Electronic components are installed on circuit boards by pick and place machines. Holes for interconnecting circuits on the board, for fastening means and the like are also automatically drilled by high speed, computer driven drilling machines which rapidly position the board, hold it in position and drill the necessary holes.
Problems arise when a drill bit breaks during the drilling operation and goes undetected. If the broken drill is not detected, the drilling machine will attempt to continue drilling holes with the broken drill. If the drill breakage occurs only a small distance from the drill tip, the drilling machine will continue to drill with the damaged drill bit, which will destroy the board or, at best, produce very poor quality holes. This may require the defective circuit board to be discarded. If the drill breakage occurs at a greater distance from the drill tip, the resultant circuit board will lack subsequent holes which were to be created in the drilling process by the damaged drill bit. A defective board which lacks certain holes may be recognized during a post-inspection process and the missing holes may be drilled. However, this correction causes a significant reduction in productivity.
While noncontact detection systems have been available, conventional noncontact systems are typically highly affected by environmental conditions and are unable to be conveniently located on the drilling system. Noncontact detection systems of the past have been based on LED or microwave technology, both highly susceptible to environmental conditions. Prior, noncontact detection systems have not utilized the benefits of the continuous repetitive movement of the object to eliminate error, interference and the effects of harsh changing environmental conditions.
Also, available detectors are required to be located very close to the object and at an angle normal to the objects motion. Generally, this is a poor location and subjects the detection device to difficult operating conditions. In a drilling machine operation, the traditional light detection systems must be placed in a ballistic path of the debris generated by the drill increasing interferences and device errors.
Past detection systems utilizing light occlusion require the object to be directly in the path of the light. On drilling systems, this necessitates the retraction of the drill bit beyond the pressure plate and fully into the pressure foot assembly. This full retraction after each drilled hole greatly decreases the efficiency of the drilling machine. Also such a system must detect small fractional signal changes, particularly for drills with smaller diameters.
Noncontact microwave detection systems operate on the principle that the presence of a continuous target, such as a fine drill, in or near the end of the waveguide at a predetermined location produces a change in the standing wave ratio (SWR) of the transmitted signal. This change in the SWR is capable of being detected by a receiver to provide a signal that is indicative of the target condition. The microwave systems do not operate like a doppler radar since only the D.C. output signal is utilized by the detection system. The frequency component of the output signal is not used by the comparator. The creation of a resonant microwave cavity and the detection of objects entering the cavity by a change in the standing wave ratio is effected by nearly any foreign object entering the resonant cavity. Thus, the system is prone to interference from debris. The microwave detection systems are also subject to interference from the copper shavings produced during the drilling of the copper layers in printed circuit boards, known as "drill-wrap," and system to system variations.
The microwave system is sensitive to precisely the target that is being detected. Thus in drilling systems, the detection device must be recalibrated for each drill diameter. This can be highly inefficient for a drilling system which engages drill bits of varying diameters via an automatic changer.
All elements in or surrounding the microwave cavity must remain constant. The components making up the cavity can not be moved without making changes or adjustment to the microwave detection device. The microwave detection system is not easily adaptable from system to system and adjustments or variations may be needed depending on the objects in and around the cavity created by each host system.
The microwave detection systems produce small signal changes from the object-present state to the object-not-present state which are analyzed by the system's comparator. Using a small signal change greatly increases the possibility of error by the microwave system.
What has been needed is a robust system which is substantially immune to environmental conditions and host system designs, a system which is flexible as to location and object to be detected and which is quick and accurate. The present invention is designed to address these needs.